Marine heatwave leaves warm water in its wake along the South Coast

A marine heatwave has passed south along the coast over the past month and more are predicted, with sea temperatures forecast to be up to 2.5 degrees warmer than usual this summer.

After a giant warm eddy formed about 50km off the coast of Sydney, the marine heatwave for the NSW South Coast was declared by the CSIRO and had produced temperatures up to 23 degrees in October.

It has since reached waters off Eden on the Far South Coast and into Bass Strait, and left unseasonably warm waters in its wake for the Illawarra.

Last month it was blamed for the deaths of large numbers of shearwaters which washed up on the region's beaches.

Ocean dynamics expert Professor Moninya Roughan from the University of NSW said the coastal waters were warmer than the average for this time of year and had bee/n up to about 23 degrees.

"That's what a marine heatwave is - it's where the ocean is above the average for that day," she said.

"It's relative to where you are and what you're experiencing at that point in time on that day.

"So as we push into December, 22, 23 degrees is not as unusual, but this one would have started in October and that's what was so unusual."

Professor Roughan said it was likely we would see more marine heatwaves this summer, and these pose risks to species and habitats that can't escape the warmer temperatures such as marine plants.

"We're expecting the ocean temperatures to be warm for the whole of the summer," she said.

"The forecasts that we have all suggest that there will be marine heatwaves [off] NSW and Tasmania and the Tasman Sea for much of the summer."

Professor Roughan said the heatwaves occur within a context of warming oceans.

"By definition they're extremes, and so extremes are in that 10 percentile range or 90 percentile range," she said.

"They're supposed to appear 10 per cent of the time, but they're appearing more and more often and they're becoming more intense. So we've got a trend of increasing temperatures in the sea and those extremes are getting warmer."

The NSW Government last week warned of the heatwave's significance to marine life and said managing their impact would be a priority over the coming months and years.

Health Minister and Minister for the Illawarra and South Coast Ryan Park said marine heatwaves were becoming worse because of climate change.

"Marine heatwaves have been expected across the south-eastern coast of Australia this 2023/24 summer as warming oceans are making marine heatwaves hotter, longer and more frequent," he said.

"The NSW Government in collaboration with key agencies and stakeholders has developed a response plan to increase marine monitoring to better understand the impact of such events.

'Early detection is vital to managing our marine environments and ensure a healthy coast line is passed on to future generations."

Minister for Agriculture Tara Moriarty said the government was undertaking "pioneering research" to increase resilience to changing ocean conditions.

"We know our oceans are warming, but it's not just an increase in mean ocean temperatures that we need to be concerned about, we also need to plan and prepare for more frequent temperature extremes," she said.

"Marine heatwaves can negatively impact a range of species and key habitats that can't escape the warmer temperatures."